點解我粒CPU成日都處於100%高使用率ge?

Maybe there's some other software running in the background i.e. windows update, maybe got spyware. But no one can give you a correct answer here coz we do not have access to your computer.

Try the following steps:

1. open task manager.
2. see if there's any suspicious files that are running.
3. google the file name of those suspicious files and see if they are harmful to your computer.
4. get rid of them.

your situation is rather unusual for a normal PC computer.

if you got 128MB or 256MB RAM memory, then it is explanable. with such limited memory running Windows XP usually got a few 50 - 80MB RAM physical memory left. and if you are surfing some heavy websites and listening to MP3 or music using WMP or other players may eat up a lot memory. then the CPU will be busy doing the page faults which causes high CPU usage.

however, if you got 512MB memory then it is really malicious. you may got virus or trojan horses infected in your computer.

in any case, please run the HiJackThis.exe and send the result log to me for inspection and diagnosis. download the HiJackThis here.
Trend Micro HijackThis 1.99.1 Download Now (207.86K) Tested spyware free

really want to help you out, if possible, tell me your PC configuration [CPU,RAM,VGA,MB info]